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 includes north-west and south-west France, Spain, Italy, the Alps, the Fichtelgebirge, east Thuringia, Harz and Rhenish Mountains.

.—Sir R. I. Murchison, Silurian System (1839) and Siluria (1854, 1867); A. Sedgwick, Synopsis of the Classification of the British Palaeozoic Rocks (1855); J. Barrande, Système silurien du centre de la Bohême (1852–1887); J. J. Bigsby, Thesaurus Siluricus (London, 1868); J. E. Marr, The Classification of the Cambrian and Silurian Rocks (Cambridge, 1883); Charles Lapworth, “On the Geological Distribution of the Rhabdophora,” ''Annals and Mag. Nat.'' Hist. ser. 5, vols. iii., iv., v., vi. (1879–1880); B. N. Peach, J. Horne, J. J. H. Teall, “The Silurian Rocks of Great Britain,” vol. i., Scotland, ''Mem. Geol. Survey'' (1899); F. Frech and others, “Lethaea geognostica,” Theil i. Band 2 (Lethaea palaeozoica) (Stuttgart, 1897–1902); Sir A. Geikie, Text-book of Geology (4th ed., 1903); and for recent papers. Geological Literature, Geol. Soc. (London, annual). See also and.

ORDU (anc. Cotyora, where the “Ten Thousand” embarked for home), a town on the N. coast of Asia Minor, between Samsun and Kerasund, connected with Zara, and so with Sivas, by a carriage road, and with Constantinople and Trebizond by steamer. Pop. about 6000, more than half Christian. Ordu has exceptionally good Greek schools, and a growing trade in filberts.

 ORDUIN-NASHCHOKIN, ATHANASY LAVRENTEVICH (?–1680), Russian statesman, was the son of a poor official at Pskov, who saw to it that his son was taught Latin, German and mathematics. Athanasy began his public career in 1642 as one of the delineators of the new Russo-Swedish frontier after the peace of Stolbova. Even then he had a great reputation at Moscow as one who thoroughly understood “German ways and things.” He was one of the first Muscovites who diligently collected foreign books, and we hear of as many as sixty-nine Latin works being sent to him at one time from abroad. He attracted the attention of the young tsar Alexius by his resourcefulness during the Pskov rebellion of 1650, which he succeeded in localizing by personal influence. At the beginning of the Swedish War, Orduin was appointed to a high command, in which he displayed striking ability. In 1657 he was appointed minister-plenipotentiary to treat with the Swedes on the Narova river. He was the only Russian statesman of the day with sufficient foresight to grasp the fact that the Baltic seaboard, or even a part of it, was worth more to Muscovy than ten times the same amount of territory in Lithuania, and, despite ignorant jealousy of his colleagues, succeeded (Dec. 1658) in concluding a three-years’ truce whereby the Muscovites were left in possession of all their conquests in Livonia. In 1660 he was sent as plenipotentiary to a second congress, to convert the truce of 1658 into a permanent peace. He advised that the truce with Sweden should be prolonged and Charles II. of England invited to mediate a northern peace. Finally he laid stress upon the immense importance of Livonia for the development of Russian trade. On being overruled he retired from the negotiations. He was the chief plenipotentiary at the abortive congress of Durovicha, which met in 1664, to terminate the Russo-Polish War; and it was due in no small measure to his superior ability and great tenacity of purpose that Russia succeeded in concluding with Poland the advantageous truce of Andrussowo (Feb. 11, 1667). On his return to Russia he was created a boyar of the first class and entrusted with the direction of the foreign office, with the title of “Guardian of the great Tsarish Seal and Director of the great Imperial Offices.” He was, in fact, the first Russian chancellor. It was Orduin who first abolished the onerous system of tolls on exports and imports, and established a combination of native merchants for promoting direct commercial relations between Sweden and Russia. He also set on foot a postal system between Muscovy, Courland and Poland, and introduced gazettes and bills of exchange into Russia. With his name, too, is associated the building of the first Russian merchant-vessels on the Dvina and Volga. But his whole official career was a constant struggle with narrow routine and personal jealousy on the part of the boyars and clerks of the council. He was last employed in the negotiations for confirming the truce of Andrussowo (September 1669; March 1670). In January 1671 we hear of him as in attendance upon the tsar on the occasion of his second marriage; but in February the same year he was dismissed, and withdrew to the Kruipetsky monastery near Kiev, where he took the tonsure under the name of Antony, and occupied himself with good works till his death in 1680. In many things he anticipated Peter the Great. He was absolutely incorruptible, thus standing, morally as well as intellectually, far above the level of his age.

 ÖREBRO, a town of Sweden, capital of the district län) of Örebro, lying on both banks of the Svartå a mile above its entrance into Lake Hjelmar, 135 m. W. of Stockholm by rail. Pop. (1900), 22,013. In great part rebuilt since a fire in 1854, it has a modern appearance. An ancient castle, however, with four round towers, remains on an island in the stream. It is used as a museum. There may be mentioned also the church of St Nicholas, of the 13th century; and the King’s House (Kungsstuga), an old and picturesque timber building. In front of the modern town hall stands a statue, by Karl Gustav Qvarnström (1810–1867), of the patriot Engelbrecht (d. 1436), who was born here. The Swedish reformers of the 16th century, Olaus and Laurentius Petri, are commemorated by an obelisk. Örebro is in close connexion with the iron-mining district of central Sweden; it has mechanical works and a technical college. A large trade is carried on, by way of the Orebro canal and lakes Hjelmar and Mälar, with Stockholm.

Örebro was in existence in the 11th century. Its castle, erected by Birger Jarl in the 13th century, played an important part in the early annals of Sweden; and no fewer than twenty diets or important assemblies were held either in the castle or in the town. Such were the Örebro concilium of 1537, the diet of 1540 in which the crown was declared hereditary, and that of 1810 when Bernadotte was elected crown prince.

 ORE-DRESSING, one of the principal processes in the work of (q.v.). When the miner hoists his ore to the surface, the contained metal may be either in the native uncombined state, as, for example, native gold, native silver, native copper, or combined with other substances forming minerals of more or less complex composition, as, for example, telluride of gold, sulphide of silver, sulphide of copper. In both cases the valuable mineral is always associated with minerals of no value. The province of the ore-dresser is to separate the “values” from the waste—for example, quartz, felspar, calcite—by mechanical means, obtaining thereby “concentrates” and “tailings.” The province of the metallurgist is to extract the pure metal from the concentrates by chemical means, with or without the aid of heat. There are also a number of non-metallic minerals which do not have any value, or at best do not reach their highest value until they have been subjected to some form of mechanical preparation; among them are diamonds, graphite, corundum, garnet, asbestos and coal. Ore-dressing, for the purposes of this article, may be divided into three parts: (1) properties of minerals which render aid in their separation; (2) simple operations; (3) operations combined to form processes or mills.

1. The specific gravity of minerals varies greatly, some being heavy, others light. The rate of settling in water is affected by the specific gravity in this way: of two particles of the same size but different specific gravity, the heavier settles more rapidly than the lighter, while of two particles of different specific gravity which settle at the same rate in water, that of higher specific gravity is of smaller diameter than the other. The same statements are true in regard to settling in air, and in regard to momentum in air when the particles are thrown out in a horizontal direction. Colour, lustre and fracture are of especial value in hand-picking, to aid the eye in selecting the mineral sought. Instances are, of colours, the white of quartz, the pale straw colour of felspar, the dull yellow of limonite, the brass yellow of chalcopyrite, the pale metallic yellow of pyrite; of lustres, the vitreous of quartz, the adamantine of diamond and cerussite, the resinous of blende, the earthy of limonite, and the metallic of pyrite; and of fractures, the cleavage planes of felspar and galena, the conchoidal fracture of quartz and pyrite, the granular of some forms of magnetite and blende. Magnetism is a most direct and simple method of separating minerals where it is available. The discovery that by the use of electro-magnets of great